How to decorate an attic with sloped ceilings and a dormer?

These spaces are a lot of fun! You can draw a line (figuratively) from ceiling to floor where it's tall enough to walk or sit and place your furniture in that area. Short bookcases will fit behind thats area. Wall decor needs to be light weight and anchored both top and bottom. Strings of x-mas lights can be hung in the unusable areas for decor and ambience. My daughter's apt is like this and it's adorable

Here are some ideas to help you visualize:

https://www.diynetwork.com/how-to/rooms-and-spaces/storage-space/amazing-attic-remodels-pictures

https://www.pinterest.com/explore/attic-spaces/

The pictures are of my grandson's room at my house years ago. Yes, he loved bright green and picked the color himself, LOL!!

26 examples here: http://www.architectureartdesigns.com/26-brilliant-bedroom-designs-ideas-with-sloped-ceiling/

How do you plan to use this space once it is decorated? Bedroom, crafts room, TV room, ....?

You could put something like a dresser in the angled area since you can't stand there.